On Thu, Apr 30, 2009 at 9:07 PM, Robert W. Anderson < anderson...@poptop.llnl.gov> wrote:
> Fergus Henderson wrote: > >> The include server could be the bottleneck. What's the CPU usage for the >> include server process? >> Or it could be disk I/O. Try iostat or vmstat to profile that. >> > > After some more experimentation, I think I may have a clue what's going on > here. I think I may be getting bound up in context switching costs, both in > my single node performance and on localhost using distcc. > > On a single node: > > -j4: 24 million context switches @ 10us = 4m > -j8: 96 million context switches @ 10us = 16m > ... > I don't understand why doubling the job count increases context switching by > 4x. Any insights appreciated. > You mentioned earlier that your host has four processors. With -j4, you have number of processors = number of jobs, so to a first approximation the only context switches should be switching between user mode and kernel mode for system calls, I/O or paging. With -j8, you now have two jobs for each processor, so you're doing additional context switches for time slicing between different user mode processes. Depending on the time slice, that could be arbitrarily higher. Cheers, Fergus. -- Fergus Henderson <fergus.hender...@gmail.com>
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